As a substrate for flat panel displays such as liquid-crystal displays, an aluminosilicate-type glass substrate has been widely used. The glass substrate for use in this application is required to have a small thermal shrinkage. Specifically, since a thin-film electric circuit is formed on the glass substrate, the glass substrate undergoes film formation heat treatment, patterning and the like treatment, and in these treatments, the glass substrate is exposed to a high temperature. At that time, structural relaxation occurs, and its volume thereby shrinks. When the thermal shrinkage is large, then the circuit pattern formed on the glass substrate may deviate from the initially planned one, thereby having a fatal defect in that it could not maintain electric properties.
There has been increasing a demand for high-accuracy high-definition flat panel displays year by year, and next-generation displays that are considered as hopeful ones satisfying the requirement are liquid-crystal display devices and organic EL devices to be driven by low-temperature p-SiTFT. In these displays, the heat treatment temperature in forming the low-temperature p-SiTFT on a substrate is a high temperature of from 450 to 600° C. or so, and the circuit pattern is finer. Accordingly, the glass substrate for use in this application is especially required to have a small thermal shrinkage.
Heretofore, the glass substrate of this type is formed according to a float method or a down draw method typically represented by an overflow down draw method. The float method is a method of casting a molten glass onto a molten tin (float bath) and stretching it in the horizontal direction to form the glass into a sheet. According to the method, a glass ribbon is formed on the float bath, and then the glass ribbon is annealed (on-line annealed) in a long annealing furnace having a length of 50 m or more. Accordingly, the glass substrate formed according to the float method is characterized by having a small thermal shrinkage. However, the float method has some disadvantages in that it is difficult to reduce the thickness of the glass substrate and, in addition, the glass substrate must be polished to remove the tin adhering to the glass surface.
On the other hand, the down draw method is a generic term for a forming method of drawing a glass in the vertical downward direction to form it into a sheet. For example, in the overflow down draw method that is at present widely used, a molten glass is introduced into the top of a drainpipe-like refractory (forming body) having a nearly wedge-shaped cross section, and the glass is made to overflow out from both sides thereof to flow down along the side face, and the two streams are joined together at the lower end of the refractory and stretched downward to form the glass into a sheet. The down draw method is advantageous in that glass can be formed into thin sheets. Further, in the overflow down draw method, the glass surface is not contacted with any other than air, another advantage of the method is that a glass substrate of high surface quality can be obtained even in an unpolished state. However, in the down draw method, an annealing furnace is provided just below the forming body, and therefore it is in fact impossible to dispose a long annealing furnace like in the float method. Accordingly, the annealing furnace is necessarily short, or that is, the cooling rate in the annealing furnace is high and a glass is solidified in a rapidly-cooled state, and therefore the method is problematic in that a glass substrate having a small thermal shrinkage could not be obtained.
In that situation, for use of the down-draw formed glass substrate for application to low-temperature p-SiTFT substrates and the like, it must be reheated (off-line annealed) to promote the structure relaxation of glass to thereby reduce the thermal shrinkage thereof. The reheating treatment includes, for example, once heating the glass substrate up to a temperature falling within a glass transition range (near the strain point or annealing point) higher than the heating temperature in device production, then holding it at the temperature for a predetermined period of time, thereafter annealed to a temperature lower by about 200° C. than the strain point, and then rapidly cooled at a cooling rate at which the glass is not broken.
Patent Reference 1: JP-A-10-53427
Patent Reference 2: JP-A-10-53426
Patent Reference 3: JP-A-2007-186406